Method and apparatus for producing clean reducing gases from coke oven gas

ABSTRACT

A process and system for producing reducing gases are disclosed, wherein volatile components derived from coal are transformed into reducing gases suitable for utilization as synthesis gas, as a reducing agent for the direct reduction of iron ores and/or as a clean fuel.

FIELD OF THE INVENTION

The invention refers to a process and system for producing reducing gases mainly composed of hydrogen and carbon monoxide from coke-making operations where coal is transformed into metallurgical coke, and more particularly to a process wherein the volatile components derived from coal are transformed into reducing gases suitable for chemical utilization as synthesis gas, as a reducing agent for direct reduction of iron ores and/or as a clean fuel.

BACKGROUND OF THE INVENTION

It is known that in the process of producing metallurgical coke, coal is heated in order to eliminate most of the volatile components and preserving mostly the carbon structure. Coke is thereby provided with the physical and chemical properties which make it fit for providing energy and burden support in blast furnaces. Volatile matter of coal comprises a number of compounds which are distilled in coke ovens constituting what is known as coke oven gas. The volume and composition of coke oven gas depends on the characteristics of the coal utilized but typically untreated coke oven gas comprises about 44% of water, about 29% of hydrogen, about 3% of carbon monoxide, about 13% of methane and many impurities as ammonia, sulfur, benzene-type compounds, etc.

Since coke oven gas has a high calorific value, it is utilized mostly for heating purposes in steel plants. Typically, coke oven gas is cooled, cleaned and treated in a number of chemical processes for separating valuable compounds such as ammonia and other petrochemicals and for removing sulfur before the gas is finally burned. Cleaning of coke oven gas require's a complex and costly chemical plant.

A number of proposals are found in patents and other technical literature for utilizing coke oven gas after cleaning, alone or in combination with other gases, for iron reduction purposes or for steam and electricity generation.

U.S. Pat. No. 4,178,266 describes a process for conveying hot crude coke oven gas generated in coke ovens to a position of utilization while preventing condensation of higher hydrocarbons. This patent teaches to increase the temperature of the coke oven gas by injecting an oxygen-containing gas into the coke oven gas stream so that it is partially combusted. Such injection of oxygen may increase the temperature of the mixture to about 950° C. to 1500° C. The purpose of the oxygen injection is to avoid condensation of impurities and of higher hydrocarbons, thereby preventing many problems in the gas conveying systems. This patent does not teach or suggest carrying out a partial combustion of coke oven gas for producing a high-quality reducing gas.

British Patent Specification No. 1,566,970 describes a process for the treatment of coke oven gas. This patent recognizes the value of performing a partial combustion of coke oven gas in order to produce a reducing gas useful for direct reduction of iron ores. The partial combustion transforms coke oven gas into a cracked gas rich in carbon monoxide and hydrogen. The partial combustion process of this patent however has a number of disadvantages and does not teach or suggest a process and apparatus which integrates the energy in an improved way.

U.S. Pat. No. 4,235,624 describes a method for processing coke oven gas almost identical to the method of the above-referred British patent 1,566,970. This patent does not teach or suggest the integration of thermal energy in the treatment and cleaning of coke oven gas comprising a partial combustion step as the present invention.

U.S. Pat. No. 4,235,624 describes a method for processing coke oven gas almost identical to the method of the above-referred British patent 1,566,970. Although this patent teaches generally that the coke oven gas may be used as a reducing gas in an iron ore reduction shaft furnace, no detail is provided about a preferred manner of utilizing the integration of the thermal energy that the coke oven gas contains. As a matter of fact, the claims of this patent specify that the invention lies in the utilization of hot coke oven gas, that means that the coke oven gas is fed to the reactor without cooling it before its introduction into the shaft reactor. This process scheme has a number of disadvantages since all equipment involved in the handling and conduction of the coke oven gas from the coke ovens to the shaft reactor must be prepared for high-temperature operation.

In contrast, the present invention provides a method and apparatus with a practical and economical way of using the coke oven gas by cooling it immediately after the partial combustion cracking and also utilizing the heat produced in said partial combustion for at least two specific purposes which are critical for the operation of the reduction reactor: e.g. for the CO2 removal unit of the recycled reducing gas to the shaft reactor, and also for the sulfur removal unit needed for cleaning the coke oven gas prior to its utilization.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a process and system for treating coke oven gas in order to produce a reducing gas comprising hydrogen and carbon monoxide, particularly useful for the direct reduction of iron ores.

It is another object of the present invention to provide a process and system for treating coke oven gas and producing a synthesis gas useful as raw material in chemical processes or for producing heat, steam, electricity or power avoiding the complex and expensive cleaning processes normally utilized in coking plants.

Other objects of the invention will be pointed out in this specification or will be evident to those skilled in the art.

SUMMARY OF THE INVENTION

The present invention provides a method of treating hot crude coke oven gas produced in coke ovens comprising: mixing said hot crude coke oven gas with an oxygen-containing gas in order to carry out a partial combustion of said coke oven gas producing a reducing gas having a high content of hydrogen and carbon monoxide at a temperature higher than about 1000° C.; passing said hot reducing gas through a heat exchanger in order to produce high temperature steam; utilizing a first portion of said steam to produce electricity; removing sulfur compounds and other impurities of said reducing gas in a sulfur removing unit; and utilizing a second portion of said steam in said sulfur removing unit.

In a preferred embodiment of the invention the method further includes an additional step of utilizing at least a portion of said clean reducing gas in a direct reduction plant comprising a direct reduction reactor, a CO2 removal unit and a gas heater.

In another preferred embodiment of the invention the method further includes the additional step of utilizing a third portion of said exhausted steam in said CO2 removal unit of said direct reduction plant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process diagram of a preferred embodiment of the invention.

FIG. 2 illustrates an example of the process showing, in a schematic process diagram similar to FIG. 1, values of process parameters.

FIG. 3 is a table supplementing the information of process streams shown in FIG. 2.

FIG. 4 also illustrates another example of the process showing, in a schematic process diagram of a direct reduction plant, values of process parameters operating according to the coke oven gas cleaning method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to the accompanying drawings, it being understood that many changes and modifications may be made by those skilled in the art without departing from the spirit of the invention, which is defined by the appended claims.

With reference to FIG. 1, numeral 10 designates a battery of coke ovens, to which coal 12 is charged and heated therein by combustion of gases 14 and air 16. Coke 18 is discharged from ovens 10 in a manner known in the art and hot crude coke oven gas 20 is produced typically at a temperature of about 600° C. degrees to 700° C.

Hot crude coke oven gas 20 is then mixed with an oxygen-containing gas 22 which may be produced in an air separation plant 24 from air 26. The air separation plant 24 may be of the cryogenic or a PSA-type since the purity of oxygen is not critical for this application. The process may also operate with oxygen-enriched air, although the quality of the resulting reducing gas is affected by the amount of nitrogen from said air feed.

Hot crude coke oven gas 20 at a temperature of about 650° C. is subject to a partial combustion with oxygen 22 in combustion chamber 28 reaching temperatures above about 1000° C. At this temperature all organic compounds are converted into a high temperature reducing gas 30 mainly composed of a mixture of hydrogen and carbon monoxide, carbon dioxide, methane and water.

Reducing gas 30 is passed through a heat exchanger 32 where it heats steam 34 from steam drum 36 and super-heated steam 38 is utilized in turbine 40 to produce electricity in electric generator 42. Electricity is fed through line 44 and is used in air separation plant 24 to produce oxygen for the partial combustion of coke oven gas.

A first portion 50 of exhausted steam 46 from turbine 40 is used in sulfur separation unit 48 and a second portion 52 is used in the CO2 separation unit 54 of a direct reduction plant.

After passing through heat exchanger 32, the reducing gas is fed to a boiler 56 where steam 58 is produced from water 60 and is held in steam drum 36. From boiler 56, reducing gas 62 is finally quenched by direct contact with water in cooler 64. Condensed water is withdrawn through pipe 66. Cold reducing gas 68 is then fed to blower 70 and then led through pipe 72 to be treated in a sulfur removal unit 48 in a manner known in the art producing solid sulfur 74 and a stream of clean reducing gas 76.

A portion of the reducing gas 76 is used for heating the coke ovens through pipe 14 and another portion 78 is compressed in compressor 80 to be injected to the reduction gas loop of a direct reduction reactor 82. Iron oxides bearing particles 84, for example iron ore pellets, lumps or mixtures thereof, are fed to reduction reactor 82 and descend by gravity through said reactor where they are contacted with high temperature reducing gases 86, for example above 900° C. whereby iron oxides are reduced to metallic iron, known as DRI or sponge iron 88, and discharged from the lower part of the reduction reactor 82 to be utilized in steelmaking operations.

Reacted reducing gas 90 is withdrawn from reactor 82 and cooled down in cooler 92 where water is removed from the gas by condensation a portion of the cooled gas 94 is recycled to the reduction reactor and another portion 96 may be vented from the reduction loop and used for example in gas heater 98. The recycled reducing gas portion 94 is compressed in compressor 100 and fed to a carbon dioxide removal unit 54 where CO2 is removed from the system thus regenerating the reducing potential of the recycled gas by elimination of water 102 and CO2 104 which are the main products of the reduction reactions carried out in reactor 82.

EXAMPLE

A computer-model calculation of a plant incorporating the invention was made and the results are illustrated in FIGS. 2, 3 and 4. This example clearly shows the advantages of the invention regarding its application for obtaining a reducing gas for producing good-quality direct reduced iron. 

1. A method of treating hot crude coke oven gas from a coke oven to produce a clean reducing gas, comprising partially combusting the hot crude coke oven gas at a temperature greater than about 1000° C. by mixing the hot crude coke oven gas with an oxygen-containing gas to produce a hot reducing gas having a high content of hydrogen and carbon monoxide; producing high temperature steam by passing the hot reducing gas through a heat exchanger; utilizing a first portion of the steam to produce electricity; and utilizing a second portion of the steam in a sulfur removing unit to remove sulfur compounds and other impurities from the reducing gas to produce the clean reducing gas.
 2. The method of claim 1, further comprising utilizing at least a portion of the clean reducing gas in a direct reduction plant comprising a direct reduction reactor, a CO₂ removal unit and a gas heater.
 3. The method of claim 2, further comprising utilizing a third portion of said steam in the CO₂ removal unit of the direct reduction plant.
 4. The method of claim 3, wherein the direct reduction plant is used to directly reduce iron ore.
 5. The method of claim 2, wherein a portion of the clean reducing gas is used to heat the coke oven.
 6. The method of claim 5, wherein another portion of the clean reducing gas is injected into the direct reduction reactor.
 7. A method of reducing iron oxides in a shaft reduction reactor utilizing coke oven gas, comprising mixing hot crude coke oven gas with an oxygen-containing gas in order to carry out a partial combustion of said coke oven gas to produce a hot reducing gas having a high content of hydrogen and carbon monoxide at a temperature higher than about 1000° C.; passing said hot reducing gas through a heat exchanger in order to produce high temperature steam; utilizing a first portion of said steam in a sulfur removal unit for removing sulfur compounds and other impurities of said reducing gas; and utilizing a second portion of said steam in a CO2 removal unit where exhausted reducing gas is treated to remove carbon dioxide before being recycled into said shaft reduction reactor. 